Pulse width modulated inverters used in three phase AC power supplies are usually equipped with harmonic filters so that the output voltages are acceptable to the applied loads. Harmonic content is ordinarily less than 5%. The result is a three phase sinusoidal voltage waveform output with small ripple voltages superimposed.
When various loads are applied, phase shifts in the ripple components occur such that the peak amplitude of the output voltages change as a function of applied load even though the fundamental component remains constant. This characteristic presents a problem in sensing the true average RMS value in a form usable for voltage control.
In a typical voltage-regulated inverter, the inverter output voltage is fed back to an error amplifier through a sensing signal processor which generates a sensing signal. Ideally, the sensing signal is proportional to the average or RMS amplitude of the three phase output voltages. The sensing signal is used in conjunction with a reference signal to generate a further signal which is used to control the output voltage of the inverter. Transient or ripple errors in the sensing signal cause distortion to be transmitted through the system and ultimately effects regulation of the inverter output.
Several different methods are commonly used to generate the sensing signal. In one method, each of the AC voltage outputs of the inverter is rectified and filtered to produce the sensing signal. Sensing signals generated in this manner tend to be peak sensitive and, therefore, are sensitive to changes in the characteristics of the applied load.
In a further method, the AC voltage outputs of the inverter are applied to a multiplying circuit, the output of which is subsequently filtered to generate a sensing signal which represents the RMS value of the AC output voltages. While this method produces a good steady state signal, the filtering of the signal causes a delay which prevents timely response of the sensing signal to transient changes in the sensed voltage which are produced, for example, by load switching. The delay in sensing the true AC output value is transferred as an error to the voltage control circuits and thus adversely effects the inverter output transient response characteristics.
A further method of voltage sensing is described in U.S. Pat. No. 5,452,198 entitled APPARATUS AND METHOD FOR A-C VOLTAGE SENSING filed in the name of Ralph D. Jessee. That patent describes an AC sensing circuit wherein a first integrated signal is generated by integrating a single phase AC signal during a time period between negative transitions of the AC signal. The integrated signal is reset at negative transitions of the AC signal. A second integrated signal is generated by integrating the AC signal during the time period between positive transitions of the AC signal. The second integrated signal is reset at positive transitions of the AC signal. The sensing signal is generated by taking the difference in magnitude between the first and second integrated output signals.